Method of dissolving metals and compostion therefor



Patented Aug. 18, 1953 METHOD OF DIS SOLVING' METALS AN D- COMPOSITION THEREFOR Richard Springer, Leipzig, Germany, and Walter R.. Meyer, Hamden, Conn, assignors to Enthone, Incorporated, New Haven, Conn, a corporation of Connecticut N'o Drawing. Application December-24, 1949, Se rial No. 135,037. In GermanyMay 13, 1949 1.4. Claims.

1 invention concerns dissolving metals oi the: class consisting of nickel, copper, cadmium,

silver, zinc, and gold, by chemical action alone,.

that is: to. say, without. the aid. of an electric current, and especially to stripping such. metals, that is, reducing the thickness of walls of articles made of metalsof the foregoing, list and removing; more; or less completely electroplates and other coatings of suchmetals formed on foundations of other materials.

The invention involves a. process, baths suitable when and Where exposed to the bath. Further, it.-

has been known for some time that solutions of cyanides will dissolve certain metals in the pres.- ence of air. It has been: known also. that the. addition of certain inorganic oxidizing agents; of

high oxidizing potentials to cyanide solutions,.

causes the cyanide solutions. to dissolve.- these. metals at rates greater than when such agents are absent. These agents render the solutionsunstable however, so that the solutions tend tobreak down rapidly, and in some instances the agcntsrender the solutions explosive.

In contrast with this, speaking generally, We have discovered that aqueous solutions containing both one or more of the cyanideshereinafter identified, and one or more nitro-substituted arc-- matic compounds of the class hereafter defined,

are capable of dissolving the metals of the classconsisting of nickel, copper, cadmium, zinc, silver and gold, at rates which are, respectively, greater than the rates of dissolution of those metals in solutions of cyanide alone in correspondingconcentrations, and at the same time without the unsatisfactory accompanying characteristics of the prior practices, i. e. without a toorapid breakdown of the bath or solution, at least with properly chosen proportions, without tendency of "the bath to breakdown with explosive violence,

and commonly at. least, by a proper selection of materials; without substantial attack on ferrous foundation or basis: materials that underlie the metal to bedissolved; The cyanidesare am:- moni'um cyanide and thosealkali metal cyanides and those alkaline earth cyanides which are ionizable in water to give cyanide ions and soluble in water to such an extent. that at least 2.5 grams of the cyanide radical can be dissolved per liter of the solution, at least atthe temperature at which the present process-is used. For conciseness all the cyanides thus identified are referred. to herein as the alkali cyanides. The. n-itro-substituted aromatic compounds are those nitrosubstituted aromatic compounds which, in. solution in water, producev an electromotive force ofminus 0.9 volt, or a. higher voltage than this, between a sheet nickel electrode and. a saturated. potassium chloride calomel. electrode. under the following. conditions: the concentration of. the nitro-compound is its: saturation. value or is about 0.2-5 gram-mol. per liter of the solution, whichever is the smaller; there. is present in the solution 2 gram-mols of. sodium. cyanide per liter. of solution; if. the nitro-compound is an acid or otherwise tends to cause an evolution of' hydro.- cyanic gas, there is. added also suiilcient sodium: hydroxide to neutralize, or to render. the solution su-fiiciently alkaline to substantially prevent the evolution. of: hydrocyanic gas the. temperature is 80 (111 C. For example, a solution of o-nitrochlorobenzene produces under such conditions. (the solution being saturated as to this nitrocompound, and. no sodium hydroxidev being. pres,- ent), a voltage-of minus.0.9: volt. Again,.a solu tion of m-nitrobenzoic. acid produces under. the prescribed conditions (the concentration of the acid, being 42. grams per liter of. the solution and 10. grams per liter oi sodium hydroxide being H added, substantially enough toneutralize the acid to. its sodium salt), a voltage of about minus 0.56- volt, i. e. a voltage higher than that ofthe o-nitrochlorobenzene. Wherever hereinafter a voltage producible by a nitro-compound is re-- ferredto,.it is-to be understood to be a voltage producible between a sheet nickel electrode and a saturated: potassium chlorideca-lomel electrodeunder the foregoing: conditions.

Speaking generally, the present process con sists of subjecting the metal or metalsto be dissolved toan aqueous solution of one or more such cyanides: and. one or more such ultra-substituted aromaticicompounds, at. a temperature or various:

riedi out' the-higherthe. rate. of dissolution. of the: metal. However; the higher that temperature the more rapidly the solution deteriorates also. Usually a temperature between about 70 C. and about 95 C. is used; usually about 80 C. is preferred for general operations. The quantity of cyanide or cyanides used in any instance can be anywhere from that quantity which provides about 2.5 grams of the cyanide radical per liter of the solution, to sufficient to saturate the solution. However there seems to be an optimum quantity for the cyanide radical, i. e. a quantity at which the effect of the cyanide content is at a maximum, namely between about 13 grams per liter and about 105 grams per liter; usually about 52 grams per liter to about 86 grams per liter of cyanide radical, more or less, is suitable for general purposes. The quantity of the nitrocompound or nitro-compounds used in the solution can be anywhere from that quantity which will provide between about 0.155 gram of the nitro-benzo grouping, i. e.

per liter of solution and suilicient to saturate the solution with the nitro-compound or nitro-compounds. Speaking generally, the greater the quantity of the nitro-benzo group present, the higher is the rate at which the metal is dissolved. About 32 grams of the nitro-benzo group per liter of the solution gives a sufficiently high rate for general purposes usually, although at times 52 grams, or even 70 grams, per liter and the correspondingly higher rate of metal dissolution is desirable in practice. As between the nitro-benzo grouping and the cyanide radical, the quantity of the nitro-benzo grouping can be from as low as one-tenth of one percent of the quantity of the cyanide radical to a very much higher figure; theoretically, apparently, to some hundreds percent. Generally however between about 30% and about 150% is used, and between about 55% and about 90% (i. e. the quantity of the nitro-benzo grouping being between about 55% and about 90% of the quantity of the cyanide radical used) seems to be satisfactory, for most general purposes at least. When the nitro-compound used in making up the metaldissolving solution is an acid (as, of course, it may or may not be), or where the compound is one that tends to act as an acid, causing evolution of hydrocyanic gas, the solution can be neutralized, e. g. by the addition of an alkali as will be understood, or at least the solution made sufiiciently alkaline to prevent substantial evolution of hydrocyanic gas. For example, a solution in water of m-nitrobenzoic acid, 45.5 grams per liter of solution, sodium cyanide, 100 grams per liter, and sodium hydroxide, 10.8 grams per liter to neutralize, is quite a satisfactory solution. However the addition of alkali to an extent noticeably greater than that required to neutralize the solution does not inhibit the metal-dissolving action of the solution, at least to a material extent. As an example, a water solution of m-nitrobenzoic acid, 100 grams per liter of solution, potassium cyanide, 200 grains per liter, and potassium hydroxide, 50 grams per liter (the latter being about 50% more than is required to neutralize the acid) also dissolves, say, nickel at a generally satisfactory rate. In some instances, particularly where the metal is to be dissolved to a considerable depth (say to a depth of more than 0,001

inch or something of that order), or where some certain nitro-compounds are used, the rate of dissolution of the metal from a given piece, while starting initially and continuing for a time at relatively high values, may fall later to an undesirably lower value, due either, say, to a tendency to passivity or to the formation of a foreign deposit on the metal surface; in such instances the articles can be removed from the solution and rinsed with water (e. g. until any deposit on the surface is removed) and then returned to the solution; this raises the rate of dissolution again. Also in some instances agitation, i. e. movement of the solution over the surface to be dissolved or movement of the article in the solution, raises the rate of dissolution of the metal very materially, whether by preventing the deposition of reaction products on the metal surface or otherwise; an increase in the rate of metal dissolution of as high as 40% by agitation has been observed; in some instances agitation has not seemed vto hasten the dissolution however. After a required amount of the metal has been removed from an article, the article may be rinsed thoroughly in clean water to stop the action. Rinsing in an acidic rinse should be avoided unless precautions are taken, since such a rinse may cause the evolution of toxic hydrocyanic gas.

The solution deteriorates both with age and with use. After the rate of dissolution has fallen materially because of consumption of the solute, or because of dragout, the solution can be replenished, generally at least twice if the solution is used continuously from the start, say by the addition of cyanide and the nitro-compound, with or without neutralizing alkali, and say in the same proportions of these, one to the other, as in the initial solution. Subsequently however the solution deteriorates to such an extent and in such a way that it is better to discard the solution and make up a new one; this is indicated when the rate of dissolution of the metal falls to an undesirably low value and fails to rise again adequately on further replenishment.

Before the operation is begun the surface of the metal to be dissolved should be cleaned thoroughly of course, say with an alkali cleaner to remove organic dirt, and with acids to remove passive films if any are present.

As an example of the process: Take for each liter of solution to be prepared, 45.5 grams of m-nitrobenzoic acid, 10.8 grams of sodium hydroxide, and 100 grams of sodium cyanide, and dissolve them in sufficient water to make up the desired quantity of solution. To hasten complete solution of these materials, the water can be heated to, say, 75 C. and the quantity of solution measured at this temperature. However the quantities of the solids per liter of solution can be varied so widely, as appears from the foregoing, that the temperature at which these proportions are determined is not of great importance. Heat this solution or bath to about C., and while holding it at this temperature hang in it plates of cold rolled steel electroplated with nickel. Speaking generally, each plate is held in the bath until the whole of the nickel plate, or such a thickness of the plate as desired, has been dissolved away. However if the stripping action ceases on a particular plate before the nickel has been dissolved to the desired depth, remove the piece from the solution and rinse it in clean nonacidic running water, removing any deposit or residue that may be on it, and then return the piece to the metal-dissolving bath. Dissolution amass the. bath is deteriorated too; much. when; the desired amount of nickel has been removed from any piece, remone. the: piece: and. it: thor' oughly= in clean. non-acidic; waiter .v one or more. pieces; are. finished they be: replaced by new: work for-similar treatment... the rate: of: metal dissolution. falls: due:v to; dragcut: or consumption of the: solute during the. first eight hours ct. continuous; use, addi water to: the: bath as; necessary and? also add further at the: nitro-compound', hydroxide and cyanide tQi the: bath in. about; the. same. proportions to. each other as; initially.. lihis: more or less restores the. dis solution rate. Usuallyr additions. of: the nitrocompound; hydroxide and: cyanide? amounting: to:- asmuch. as. 7-5 oftheaquaniiitgofi thosematerials; with. which. the. bath. was: made up;- initially; can

be made. to; the bath du'fing the first day-:. The

dissolution rate is high. about; the; first; eight: hours use usually; and then falls; gradually so that: the second. day the rate: is. noticeably than. during the: first; dam. The. both; can be 1:82 plenished; again the second day, but after; say; the. third successive. day oi use. the. bath. will; have. deteriorated: to; such a: degree that it is well; to discard: and; make.-v up: a... fisesh bath. for fiirtheiz" work. Most low carbon; steels, cold. rolled. steels and: cast irons are. not attacked-1 by a. baths when freshly: made; accordirigly. whenthe. icon-- dation 01: basis metal underho'ng: the nickel. plate is formed by" oi:these',.the:fioundation on basis: of: the nickel, whereveritzmayb'e exposed to the.- bath, is not affected by the nickel strippingoperration; if after someuse of theibath. such: a foundation is attacked", this: indicates that the: bath. has deteriorated and should be discarded.v How'- ever, some types of high carbon steels: and; cast irons are attacked slightly by solutions or baths. made up as here described; During the metal dissolving operation a. small. amount of: cyanide; fumes is= given: oil; i'. e. to about the same extent; as with a: hot cyanide plating: solut'ion; and iiiadditionan odor of ammonia. Accordingly: eis haust: ventilation should. beproui'dedi tor thetank inwhich: the solution is held.

As other examples: Take the same acid,v hy dioxide and cyanide as in. the firstexample, butv usingv perliter of solutionv more than the? quantity of' each stated in: that: example: With: the same operations; the" results are. substarr tiall y the same as before: but the rate of. dis solution of the nickel is higher than its theifirst example. Again, take the same solutes as: before: but use per liter of solution 75 more than. the: quantity of each stated in the first examples. With. substantially the: same. operatibnsz as. be:- fore, the: results are substantially the same ex:-- cept the nickel is dissolved. at. a. still: higher rate.

As still another example: For: each lite]: oi solution tobe prepared, take 5 11.5:- grams of m-sod'ium nitro-benzoat'e and 100 grams,- of so.*- dium cyanide and dissolve: them in. suflicient; water to make up the: desired: quantity of? SGIIllL-= tion. The procedure and. operations; oi: thefirst. example. can be followed and substantially the: same results obtained.v

With such; solutions or bathsand operations as. those. justdescribed, electroplated: nickel is. dissolved at. rates. of; from; about". 0.0005= 130*;0052 or more inch: per hour: (is. e. is; reducted: im thick-- ness at such rates when the bath iSs acting; on. one side only of the plate, the rate in. any in.-

stance depending" poziirrarilsr the; conceit-trap tion: of the solutes; in. bath, the: temperature:

6 of the operations. the: age of. the bath and. extent to: which the: bath; has; been. used; and. perhaps process bywhich: the;- nickel is: elecs troplated.

As a still. further-r example:- M-ake up the. bath. containing; mv-nitrobenzoio acid, potassium: cyanide and: potassium. hydroxide: which; is describedabove, heat.- it to; C.,, and-.whileholding; at. this temperature hang plates: of electroplated nickel; in. it.. The: nickel is: dissolved? at: rates even higher thanthose. mentioned. above.

\ Nitrobenzoicacid; together with. sufiicient, 11ydroxide to neutralize. or; an. excess of the hydroxide, or. equivalentlya vnitrobenzoate. salt such asv results: from the reaction: between@ such. acid; and hydroxide seem. to be preferable. to the. othernitro-compounds, because: of. the wide; range. of stripping rates: which can be obtained by; using smaller or larger quantities. of; them. per liter of. water.. The meta. form. seems: to be; especially preferable for general use since not: only does it have a wide range of stripping; rates: but. alsmis. generally available.- and; it; tends to. have: stripping rate greater than; that. of. the

para -terms even. Forexample,underlikecondi-e tions the meta. form. has givena. stripping; rate; of. 0=00-2l6.- inch of nickelzper: hour: While; the para: form-v gave a. rate of. 0;00-149- inch, per hour;

However, other nitro-compounds are useable as appears from the foregoing. For example, and for. the purposes of. comparison; (Thefollowing figures:- of' paragraph. were obtained from; aqueous solutions containing 98 grams of. sodium cyanide per liter of solution, operation at 80 (3.,

; the grams per liter stated are grams of the respective. substances perliter of the solution, sat. sol. indicates-that. the bath or-solution was. saturated-with the respective compound, and theratefigures indicatesubstantially the initial stripping. rates; oi electroplated nickel in: frac:-- tions or an inch per hour.) Nitroanilines; e.. g. o-nitroaniline;v sat. sol-L, voltage minus 0.82, rate 0.00013 again, m-n-i-troarniline,v sat. sol.,, vo1tage-..minus 0.7-3, rate 0.000598; and again, p-ni-troaniline,. sat. sol1, voltageminus- 0.81, rate 0.00018. Nitrophenols, e. g., o-nitrophenol, 35 grams: perliter; sodium. hydroxide 1-0 grams; per liter, voltage minus 0.83, rate 000014;; again. 2-nititoresorcinol, 39' grams; per: liter, sodium. hy-

dnoxide- 1.0 grams per liter, voltage minus 0;76;

-. sodium. hydroxide 1.0. grams per liter, voltage.

minus:0.7-6, rate 0=000235s Nitrobenzene sulfonic. acid; eg. m-n-itrobenzene sulfonic. acid, 5.7. grams per. liter, sodium. hydroxide. 10- grams per liter, voltage minus. 0.65, rate.- 0100242. Nitrobenza-ldehydes; e-.. g; m-nitrobenzaldehyde, 38 grams per liter; voltagae.minus 0.70, rate 0.00062.v Also p-nitrotoluene, o-nitrochlorobenzene, and; p-nitrochlorobenzene. are useable, but they are of. rather low solubil-ities, their voltages are.

7 rather low, and their stripping. rates. are rela tively lowa. Incidentally; the m-.-nitrobenzene sulionic. acid. seems.- to; tend to, rather considerable: sent-decomposition; and with m.-nitrobenzalde-- hyde an orange colored precipitate has been noticed.

The foregoing specific matters have been. de-' voted. to the'action. on nickel. so that-theresults obtainable with; the; various materials can. be.- compared. As appears earlier abovehowever; the. same.- process: is applicable to stripping copper,

zinc, cadmium, gold and silver also. For example, an aqueous solution containing 98 grams per liter of sodium cyanide and 10 grams per liter of m-sodium nitrobenzoate, at 80 C., stripped the following metals, electroplated from standard cyanide solutions, at the following rates in inch per hour: silver, 0.000193; copper, 0.00173; cadmium, 0.00157; zinc, 0.00105; and when the benzoate concentration was 47 grams per liter: silver, 0.00525; copper, 0.00783; cadmium, 0.00766; zinc, 0.00746.

Speaking generally at least, one or more of the cyanides identified above and one or more of the nitro-compounds can be mixed in the dry state in appropriate proportions for baths for the present process, and stored and transported in this form for use when and where needed. The proportions are those appearing above of course where necessary to neutralize or prevent evolution of gas from the solution, one or more hydroxides can be included in the dry mixtures.

Itwill be understood that the invention is not limited to the details of materials and operation mentioned above, except as appears hereafter in the claims, and that the claims are intended to include equivalents of the various matters appearing in them as well as those matters specifically.

We claim:

1. A metal-dissolving composition for dissolving metals by chemical action containing at least one water-soluble alkali cyanide that is ionizable in water to the extent to yield at least 2.5 grams of cyanide radical per liter of aqueous solution, and at least one nitro-substituted aromatic compound capable of producing an electrical potential of minus 0.9 volt or higher between a sheet nickel electrode and a saturated potassium chloride calomel electrode, the nitro benzo grouping of the nitro-compound being, by weight, equal to more than one-tenth of one percent of the cyanide radical, and the composition being alkaline in reaction and being substantially free from nickel, copper, cadmium, silver, zinc and gold.

2. A metal-dissolving composition for dissolving metals by chemical action containing at least one water-soluble alkali cyanide that is ionizable in water to the extent to yield at least 2.5 grams of cyanide radical per liter of aqueous solution, and at least one nitro-substituted aromatic compound capable of producing an electrical potential of minus 0.9 volt or higher between a sheet nickel electrode and a saturated potassium chloride calomel electrode, the nitrobenzo grouping of the intro-compound being, by weight, equal to between about 30% and about 150% of the quantity of the cyanide radical, and the composition being alkaline and substantially free from nickel, copper, cadmium, silver, zinc and gold.

3. The subject matter of claim 1 characterized by the fact that the nitro-compound content consists of at least one material selected from the group consisting of nitrobenzoic acid, nitroaniline, nitrophenol, nitrobenzene sulfonic acid, nitrobenzaldehydes, and neutralized forms of any of the foregoing.

4. The subject matter of claim 1 characterized by the fact that the alkali cyanide is an alkali metal cyanide, the aromatic compound is a nitrobenzoic acid, and at least one hydroxide is included in the composition.

5. The subject matter of claim 1 characterized 8 by the fact that the cyanide is an alkali metal cyanide and the aromatic compound is m-nitrobenzoate.

6. A metal-dissolving solution of water, at least one alkail cyanide that is ionizable in water to the extent to yield at least 2.5 grams of cyanide radical per liter of solution, the quantity of the cyanide being suficient to provide the metaldissolving solution with at least 2.5 grams of cyanide radical per liter of the solution, and at least one nitro-substituted aromatic compound capable of producing an electrical potential of minus 0.9 volt or higher between a sheet nickel electrode and a saturated potassium chloride calomel electrode, the quantity of said aromatic compound being sufficient to provide the solution with at least 0.155 gram of the nitro-benzo grouping per liter of solution, and the solution being substantially free from nickel, copper, cadmium, silver, zinc and gold.

7. The method of dissolving away an exposed surface of a metal of the class consisting of nickel. copper, cadmium, silver, zinc and gold, which consists in subjecting the surface to an aqueous solution containing per liter of solution, at least about 2.5 grams of cyanide radical and at least about 0.155 gram of nitro-benzo grouping of a nitro-substituted aromatic compound capable of producing an electromotive force of at least minus 0.9 volt between a sheet nickel electrode and a saturated potassium chloride calomel electrode, in the absence of a substantial flow of electric current between said surface and the solution.

8. The subject matter of claim 7 characterized by the fact that the solution is held at a temperature between about 70 C. and about C.

9. The method of dissolving away an exposed surface of a metal of the class consisting of nickel, copper, cadmium, silver, zinc and gold, which consists in subjecting the surface to an aqueous solution containing, per liter of solution, between about 13 grams and about grams of cyanide radical and more than 0.155 gram of nitro-benzo grouping of a nitro-substituted aromatic compound capable of producing an electromotive force of at least minus 0.9 volt between a sheet nickel electrode and a saturated potassium chloride calomel electrode, in the absence of a substantial flow of electric current between said surface and the solution.

10. The subject matter of claim 9, characterized by the fact that the solution contains between about 52 grams and 86 grams of cyanide radical per liter of solution and the solution is alkaline.

11. The subject matter of claim 10 characterized by the fact that by weight the quantity of the nitrobenzo grouping is between about 30% and of the cyanide radical.

12. The method of dissolving away an exposed surface of a metal of the class consisting of nickel, copper, cadmium, silver, zinc and gold, which consists in subjecting the surface to an aqueous solution of at least one alkali cyanide, at least 5 grams per liter of solution, and at least one nitro-substituted aromatic compound, at least 2.2 grams per liter of solution, selected from the following: nitrobenzoic acid, nitroaniline, nitrophenol, nitrobenzene sulfonic acid, nitrobenzaldehydes, and neutralized forms of any of the foregoing.

13. The subject matter of claim 12 characterized by the fact that the solution is alkaline.

14. The method of dissolving away an exposed surface of a metal of the class consisting of nickel, copper, cadmium, silver, zinc and gold. by chemical action, which consists of subjecting the surface to an aqueous solution of at least one alkali cyanide, sufficient to provide between about 52 grams to 86 grams of cyanide radical per liter of solution, and at least one alkali mnitrobenzoate, sufficient to provide between about 52 grams and about 70 grams of nitro-benzo grouping per liter of solution.

RICHARD SPRINGER.

WALTER R. MEYER.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 331,810 Mutchler Dec. 8, 1885 Number Name Date Sully July 6, 1897 Bush July 9, 1907 Sperr Oct. 27, 1936 Vaughen July 6, 1937 Wernlund June 23, 1942 Lum Oct. 7, 1947 Kerridge Feb. 10, 1948 FOREIGN PATENTS Country Date Great Britain June 18, 1903 France Aug. 31, 1908 

1. A METAL-DISSOLVING COMPOSITION FOR DISSOLVING METALS BY CHEMICAL ACTION CONTAINING AT LEAST ONE WATER-SOLUBLE ALKALI CYANIDE THAT IS IONIZABLE IN WATER TO THE EXTENT TO YIELD AT LEAST 2.5 GRAMS OF CYANIDE RADICAL PER LITER OF AQUEOUS SOLUTION, AND AT LEAST ONE NITRO-SUBSTITUTED AROMATIC COMPOUND CAPABLE OF PRODUCING AN ELECTRICAL POTENTIAL OF MINUS 0.9 VOLT OR HIGHER BETWEEN A SHEET NICKEL ELECTRODE AND A SATURATED POTASSIUM CHLORIDE CALOMEL ELECTRODE, THE NITRO BENZO GROUPING OF THE NITRO-COMPOUND BEING, BY WEIGHT, EQUAL TO MORE THAN ONE-TENTH OF ONE PERCENT OF THE CYANIDE RADICAL, AND THE COMPOSITION BEING ALKALINE IN REACTION AND BEING SUBSTANTIALLY FREE FROM NICKEL, COPPER, CADMIUM, SILVER, ZINC AND GOLD. 